Robotic work tool systems, such as lawnmower systems, are often electrically powered. Since the work task, that the robotic work tool is designed to perform, is often quite power consuming solar power may not provide an adequate power supply and many robotic work tools are designed to return to a charging station to recharge its batteries between or during work sessions. Many different manners of finding the way back to the charging station are known in the field of robotic work tools. Such manners include following a boundary wire to the charging station, following a guide cable to the charging station or following a magnetic field generated at and emanating from the charging station. Such a magnetic field, which is sometimes called an F-range, may be generated by a conducting wire loop through which a current is transmitted. The current will generate a magnetic field in the loop. The magnetic field will be strongest at the centre which is placed close to the charging station and a robotic work tool may find its way back to the charging station by propelling itself towards an increasing magnetic field strength. Details on how such an F-range may be established and how the robot may be configured to follow the F-range can be found in the European Patent Application EP 1 512 054 entitled “ELECTRONIC DIRECTING SYSTEM” that discloses an arrangement and an electronic navigational control system for a self-propelling device (5), preferably a lawn-mowing robot. The system comprises at least one navigational control system (3) connected to at least one signal generator (1) and a sensing unit arranged at the self-propelling device (5). The sensing unit senses at least one, in the air medium propagating, time and space varying magnetic field, at least transmitted via the navigational control station (3) and in turn retransmits at least one signal processed by the unit to at least one driving source which contributes to the device's movements across the surface. The system comprises means by which the signal generator (1) sends a current through the navigational control station (3), the current generating the time and space varying magnetic field, whereby the sensing unit comprises means by which the device (5) is manoeuvred based on the properties of the sensed magnetic field.
However, if a work area of the robotic work tool should contain objects or other obstacles such as walls, trellises, partitions, hedges or bushes, the robotic work tool may be blocked from following the magnetic field strength and may end up running out of power before reaching the charging station. The robotic work tool may also be prevented from finding its way to the charging station if the robotic work tool is configured to follow a boundary wire until it can detect the F-range and then follow the F-range and if the charging station is placed close to boundary wire in which case the F-range will extend also beyond the boundary wire and the robotic work tool may then accidentally escape the work area that the boundary wire encloses while it follows the higher prioritized F-range.
Furthermore, the manner of following an F-range may be unsuitable in certain work areas, for example where the charging station is placed in a cramped section that has a very narrow opening and that is difficult to enter correctly unless following a guide cable.
There is thus a need for an improved manner for a robotic work tool to find the way back to the charging station with a reduced risk of running out of battery by getting blocked from reaching the charging station.